What does every young artist who wants to be great, need to know?
Becoming “great” is not something you are capable of judging. It’s something that others judge - and usually after a long, long time of you working your ass off.
What an artist does who may (or may not) end up with that appellation is to focus on two things: their craftsmanship and the expansion of their creative imagination.
Craftsmanship means you probably have to take art classes, because you have to know the basics - drawing, color relationships, composition. And you also need critiquing from a real, live and knowledgeable human being. It’s only when you know the rules, and have practiced them faithfully for many years, when they have become your automatic nature, that you can then break them and soar into the realms of surprise, improvisation and real genius.
Your creative imagination must be fed. John Schlitz’ answer is spot on. Your artist’s “life voice” is critical, but if you are to strive for excellence, it has to come from the deepest and broadest humanity you can muster. That means you need to live deeply and broadly, not just sequester yourself in your studio. What will you know about life if you do that? What in the world will you have to communicate that is worthwhile and real?
Therefore go first study. Then live and love and play and dance and sing. And paint while you do. Assiduously.
Someday, maybe, someone will call your work good.